Presently, silicon wafers, the raw material from which most micro-electronic integrated circuit chips are fabricated, are generally a planar, disk-shaped object approximately 0.025 inches thick having a diameter between 3 and 6 inches. Fabricating such a wafer begins with sawing across a cylindrically-shaped single crystal boule of extremely pure silicon material along a particular crystallographic axis to produce these thin, circular slices. Usually the wafer thus produced does not have a continuous circular periphery but rather is fabricated with one or two reference marks such as flats or notches on its periphery which establish its crystallographic orientation. These reference marks can also be used to facilitate mechanically orienting it during integrated circuit fabrication.
Before the fabrication of individual integrated circuits may commence, further processing of the silicon wafer's planar surfaces is necessary. First the rough-sawed wafer is usually lapped to remove most of a layer of stressed material formed on its planar surfaces by the sawing operation. Then the wafer will be processed in an acid etch to remove the remainder of the stress-damaged material. Finally, one entire planar surface of the wafer will usually be processed with a doping material to uniformly establish a particular desired electrical characteristic. This doped planar surface of a wafer is generally referred to as the wafer's "frontside." The other planar surface of the wafer which has not been doped is generally referred to as its "backside." This initial doping operation is generally performed on the raw silicon wafers before they are shipped to an integrated circuit factory. However, the precise character of the doping cannot be established by merely inspecting the wafer. Rather, costly analytical testing is required to determine a wafer's doping if this property is unknown. Consequently, such doped raw silicon wafers are generally shipped in carefully organized batches of wafers to permit the easy determination of their doping without resorting to analytical testing. Further, within the integrated circuit factory wafers must be handled carefully to preserve the character of their doping.
Considering the mechanical structure of a silicon wafer, it is seen that the wafer constitutes a uniformly thick circular membrane having a diameter approximately 400 times greater than its thickness. When handled manually, wafers are generally grasped at one location along their periphery with a tweezer having "duck bill shaped" tips to distribute the gripping force over a relatively large area. When thus grasped, the wafer's mechanical structure may be considered to be that of a horizontally cantilevered beam subject to the force of gravity which has a length several hundred times greater than its thickness. While silicon is a comparatively strong material, it is also rather brittle and consequently this thin membrane or long beam is relatively fragile. Therefore, during integrated circuit fabrication silicon wafers are handled carefully both to prevent scratching the planar surfaces on which the integrated circuit chips are fabricated but also to prevent chipping the wafer's circular edge. Edge chips are particularly damaging to a silicon wafer because of the high temperature processing used in integrated circuit fabrication. If a water has a chipped edge, it will frequently crack during this high temperature processing due to stresses originating at the chipped portion of its edge. In addition to requiring that the wafer be properly oriented with respect to the crystallographic axes of the silicon crystal, that the wafer be essentially stress free, that one of its planar surfaces be properly doped, that the identity of the doping be preserved, and that the wafer remain scratch and chip free, fabricating integrated circuits with a high yield of good chips requires that its doped surface be kept extremely clean.
Within the integrated circuit factory, the cleanliness required for high yield chip manufacture is obtained by performing all the fabrication operations in a clean room environment. This clean room environment is principally established and maintained by a constant laminar flow circulation of air through ultra-pure filters located in the ceiling, then downward through the volume of the room and finally out of the room through a porous floor. In time this downward laminar flow of air establishes the clean room environment by driving the contaminants from the room and/or trapping them in recesses within the room from which they cannot escape. However, those particles which inevitably remain in the clean room despite this laminar air flow and do not pass out of the room through its floor tend to settle on the horizontal surfaces within the room just as dust settles onto a polished wood table in a home.
While by most normal human standards the air in an integrated circuit factory clean room is very clean, integrated circuit manufacturers are constantly striving to reduce contamination within their clean rooms because improved cleanliness increases the yield of good integrated circuit chips. Further, each advance in integrated circuit manufacture which reduces the size of the chips or correspondingly allows the manufacture of ever more complicated chips imposes ever more stringent requirements on cleanliness to achieve an economically acceptable yield of good integrated circuit chips. Because the workers present in the clean room of an integrated circuit factory are a highly significant if not the major source of contamination, the present trend in achieving the higher levels of cleanliness required for advanced, very large scale integrated circuit manufacturing is to automate wafer processing as much as possible so fewer workers need be in the clean room environment.
This semi-automatic or even fully automatic integrated circuit manufacturing requires that the wafer processing machines be capable of transporting the disk-shaped silicon wafers. To prevent scratching the wafer's doped planar surface or chipping its edge and to support it in a low-stress condition free from excessive gravitational loading, presently available automatic wafer processing equipment is generally designed to handle silicon wafers by contacting only the wafer's comparatively large backside planar surface. While the automatic wafer handling structure incorporated into various wafer processing machines varies depending upon the particular operation to be performed and the particular devices used to perform it, presently such machines are designed to transport the wafer to locations lying only in a single, horizontal planar surface. Generally horizontal wafer tracks are used to provide rectilinear movement for the wafer between the machine's various processing stations. While a wafer is located on this wafer track, its backside generally rests on elongated, moving rubber bands or on an upwardly directed flow of air. With either rubber band or air wafer tracks it is impossible to maintain a wafer's orientation while it is being transported. Consequently, if an apparatus such as a photolithography machine employs a wafer track for handling the wafers and also requires that the wafer be held in a particular orientation during processing, then the apparatus must also include a device for properly orienting the wafer at the site where it is processed.
Between processing operations, whether performed manually or by an automatic wafer processing machine, integrated circuit wafers are generally kept in a "wafer carrier," an elongated, rectangular cassette having one open side in which a plurality of disk-shaped wafers are individually received into separate slots in a side-by-side arrangement. Automatic wafer processing machines are generally designed to accept wafers for processing directly from an input wafer carrier and are similarly designed to deliver processed wafers directly to an output wafer carrier. The device for introducing wafers into an automatic wafer processing machine or receiving them from that machine is generally an elevator type of device which includes a tower adapted to receive an entire wafer carrier with the wafer's planar surfaces oriented horizontally. This elevator apparatus then sequentially raises or lowers the wafer carrier a distance equal to the spacing between adjacent wafers to allow individually transferring wafers from the carrier to the horizontal wafer track or conversely. While this horizontal, planar surface approach to wafer handling devices avoids exposing the wafers to excessive mechanical stress and/or edge chipping, maintaining the wafer in a horizontal position during processing exposes it to the greatest possibility of contamination possible due to dust particles present in the laminar flow of air passing downward through the clean room.
Minimizing an integrated circuit wafer's exposure to contamination from dust particles present in a clean room's downwardly directed laminar air flow may be achieved by orienting the wafer's planar surfaces vertically rather than horizontally. Such vertical handling also reduces the mechanical stress of gravitational loading on the wafer because the force of gravity now acts parallel to the wafer's planar surfaces rather than perpendicular to them. At normal room atmospheric pressure, wafers may be transported by a vacuum chuck in such a vertical orientation as well as in virtually any other orientation while contacting only a wafer's backside. Further, a vacuum chuck wafer handling system is, in principle, capable of preserving a wafer's orientation as it is transported. However, the current trend in integrated circuit wafer processing is away from wet chemical processing at a normal atmospheric pressure. Rather, modern integrated circuit manufacture increasingly relies on dry processing because of its ability to delineate the very small features necessary to fabricate the components needed for very large scale integrated circuits and to fabricate structures having very high aspect ratios, i.e. the ratio of a structure's height to its width. Since such dry processing is generally performed at relatively low pressures, i.e. in a vacuum, a vacuum chuck which grips a wafer due to the pressure of the surrounding atmosphere urging the wafer toward the chuck is incapable of handling wafers within a dry processing apparatus.